BackgroundThe Japanese chaff flower, Achyranthes japonica, is used as complementary medicine to control degenerative arthritis. Although commonly used in South Korea, there has been no report of side effects. We report the first case of acute interstitial nephritis (AIN) that occurred in a woman who ingested A. japonica extract for 4 months.Case presentationA 56-year-old Korean woman was admitted for deterioration of renal function. She had general weakness and nausea for 1 month. Her initial blood urea nitrogen and serum creatinine levels were 26.3 mg/dL and 3.2 mg/dL, respectively. She acknowledged ingesting A. japonica extract for the past 4 months. Renal histology demonstrated AIN represented by immune cell infiltration into the interstitium, tubulitis, and tubular atrophy, but the glomeruli were intact. A. japonica was discontinued immediately and conservative management was started. Renal function was nearly restored to the baseline level without medication after 13 months.ConclusionThis is a rare case report of AIN associated with a pure A. japonica extract. In the case of unknown etiology of AIN, physicians should ask about the use of herbal medicines, nutraceuticals, and traditional folk medicines including A. japonica.